Analyzing your own nitrox tanks

[VsubT]

I wouldn't trust them.
They'll write the mix on a Hello Kitty sticker and then slap it on your tank !!

Too funny - you nailed that one. I used to carry a Hello Kitty notebook into meetings as an ***hole detector. You could tell a lot by how people reacted to it.

Lance

 

[elgringoperdido]

I took the PADI nitrox course this week, and it was made very clear that you always analyze your own tanks before using them. Then I was told that the analyzers are very expensive and there is usually one around to use.

The next day I got on the boat with the same shop (my instructor was on board). I used their analyzer and got a very different reading from what was marked on the tanks. A crew member came to help me and it turned out the analyzer was drifting after being calibrated. After we tried a few times and got varying readings approaching what the markings said, she told me that it was okay to use the percent marked on the tank.

That night I met a local guide for a dive and she provided the nitrox tanks. She didn't have an analyzer so I went with what she had marked on the tanks.

It seems like many of the "rules" I've been taught in scuba courses tend to be more "guidelines" when I get out in the real world. Is this one of them? What are people's practices as far as analyzing your own tanks?

The local shop that fills my tanks insists that we check the pressure and the percent O2 before we pay for them and haul them out to the car. On tropical trips where I rent tanks, I always use the shop analyzer. Even if I blend my own (I have two blenders certificates) and I do all the calculations, I check it. On technical dives where I used 100% O2 I never checked it because I always watched them fill it from a large oxygen cylinder, clearly marked. They tend to fill those as needed, not long before the dive, so I have always had the chance to watch them fill it.

The times that I have been on a boat in the tropics where the nitrox tanks were already on the boat and they just say "pick one" I calibrate the sensor myself using the ambient air. The few times I had inconsistent readings I just assumed 34% O2 and make a profile based on that. 1.4 = (0.33)(4.12) = (0.33)(1+3.12), so 31.2 meters would be my maximum depth in that case, which is about 102 feet. I say that because in places where they push nitrox, or have "free" nitrox, the blenders are typically shooting for 32% O2, and I have seen them range from 29% to 34% using trustworthy analyzers.

 

[BRT]

The local shop that fills my tanks insists that we check the pressure and the percent O2 before we pay for them and haul them out to the car. On tropical trips where I rent tanks, I always use the shop analyzer. Even if I blend my own (I have two blenders certificates) and I do all the calculations, I check it. On technical dives where I used 100% O2 I never checked it because I always watched them fill it from a large oxygen cylinder, clearly marked. They tend to fill those as needed, not long before the dive, so I have always had the chance to watch them fill it.

The times that I have been on a boat in the tropics where the nitrox tanks were already on the boat and they just say "pick one" I calibrate the sensor myself using the ambient air. The few times I had inconsistent readings I just assumed 34% O2 and make a profile based on that. 1.4 = (0.33)(4.12) = (0.33)(1+3.12), so 31.2 meters would be my maximum depth in that case, which is about 102 feet. I say that because in places where they push nitrox, or have "free" nitrox, the blenders are typically shooting for 32% O2, and I have seen them range from 29% to 34% using trustworthy analyzers.

You are more worried about oxtox than DCS?

 

[SuitableSkies]

Analysers are so cheap that I don't get why most people who dive regularly don't own one. You can even make one for next to nothing. There are tons of DIY designs out there. And with the cost of Arduinos and ESP32's these days you barely have to do anything more complicated than plugging a few wires together. An Arduino Nano, a display, an ADS1115, half a dozen jumper wires and about 30 lines of code. Probably £15 at the most from Ebay. I keep meaning to put mine up on GitHub.

The cell is the biggest investment but even then it's what? £30 a year? Or free if you dive a rebreather and changing cells regularly (or know someone who does).

Is there a particular design you'd recommend? I'd be interested in making an O2 + CO analyzer.

 

[San Diego Climber]

Both of you are missing the point.
If the shop has a dedicated tank that is kept where the analyzers are hanging on the wall - marked with 20.9% - then that tank can be trusted to be air and only air. It eliminates the opportunity for it to be a tank that has any blend other than air in it.

We’re all referring to the atmosphere i.e. (almost) guaranteed to be 20.9%, or at least more likely than even a marked “air” tank would be (which would sources by the compressor from the “big tank” anyway.)

 

[elgringoperdido]

You are more worried about oxtox than DCS?

It's just that I watch them fill the neat oxygen cylinders with my own eyes. I know it's pure. I guess things could become mislabeled, but it seems unlikely, particularly with a reactive gas like oxygen. Once it's in my hands I label it immediately. I write 100% O2 MOD 6 m on a piece of tape. And I put the cylinder in a certain spot on my gear. And I follow my shearwater using the conservative default settings and suck on that at a depth of 20 feet till it tells me I am clear. DCS is still possible, of course, but it's a risk that all divers take during any dive.

 

[tursiops]

It's just that I watch them fill the neat oxygen cylinders with my own eyes. I know it's pure. I guess things could become mislabeled, but it seems unlikely, particularly with a reactive gas like oxygen. Once it's in my hands I label it immediately. I write 100% O2 MOD 6 m on a piece of tape. And I put the cylinder in a certain spot on my gear. And I follow my shearwater using the conservative default settings and suck on that at a depth of 20 feet till it tells me I am clear. DCS is still possible, of course, but it's a risk that all divers take during any dive.

I think you missed the point.
You said, regarding nitrox:

The few times I had inconsistent readings I just assumed 34% O2 and make a profile based on that.

You were asked:

You are more worried about oxtox than DCS?

The point is, you probably should use the lesser of your variable Nitrox readings to set your O2 on your computer, and the greater of them to choose your MOD.

 

[elgringoperdido]

The point is, you probably should use the lesser of your variable Nitrox readings to set your O2 on your computer, and the greater of them to choose your MOD.

But in fact that is exactly what I did. I thought I made it clear that the profile was based on an overestimation (perhaps) of the O2. For setting the dive computer I generally chose a lower reading if inconsistent readings presented. When I mentioned the "conservative default" settings on the computer, I referred to the algorithm, not to the user-defined gas mixtures. I input those before every dive.

Perhaps I did miss his point. I really took the question at face value: do I worry more about DCS or about oxygen toxicity? I"m not sure of the answer because I think they are both problems that should be avoided, but my gut says DCS is probably a more common problem. Don't have any statistics to back that up, though. I know that DCS can occur even if you use a conservative algorithm and religiously follow your computer or dive table and even if your analyzer was calibrated by Geordi Laforge himself. It occurs sometimes in healthy, sober, well-hydrated divers in relatively shallow waters.

As for the pure O2, I don't crack that one till I reach 20 feet. But if all my other cylinders failed at depth I'd not hesitate to breathe it. And if they all failed I'd get to the surface as quickly as I could, ignoring my computer's beeping. It is possible in some cases to recover from DCS. It is never possible to recover from death.

Feel free to disagree with me. My advice to the OP is clear: the "rule" is really a good one. Follow it. If you get in a situation where you cannot follow the rule (such as I described: e.g., on a boat in the tropics, don't have your own analyzer, the one given you is not trustworthy, etc.) then err on the conservative side: dive a bit more shallow and short. This does not increase the probability of DCS in order to avoid oxtox, nor does it incur the opposite cost. It can help ameliorate both problems, although no protocol will eliminate the risk completely.

 

[tursiops]

in places where they push nitrox, or have "free" nitrox, the blenders are typically shooting for 32% O2, and I have seen them range from 29% to 34% using trustworthy analyzers.

So why do you set your computer on 34, if the measured ranges are 29-34?

 

[Norwegian Cave Diver]

We’re all referring to the atmosphere i.e. (almost) guaranteed to be 20.9%, or at least more likely than even a marked “air” tank would be (which would sources by the compressor from the “big tank” anyway.)

What do you set an analyzer to?
A tank specifically marked 20.9?
Or
A random tank?
Or
Wave your analyzer in the air?